IF Filter & Amplifier Question

[Ow@i$]

I wonder after a mixer stage of AM receiver when an IF of 455 kHz is obtained can it be filtered and amplified using an op amp ( Low Pass filter and non-inverting amplifier)???
However I have never seen OPamp being used in receiver they use typical RC combinations and transistors for amplification. This whole can be easily done with one single Opamp.
kindly tell why not to use opamp????

 

[chuckey]

one importent function of the IF amplifier is to provide AGC (Automatic Gain Control). This is missing from an OPamp, so an additional bit of circuitry would be needed for this function. FWIW. the DC across the detector diode is fed back to reduce the currents in the IF amplifiers which use a transistor which is designed for their gain to fall with reducing Ic. The AGC range for a typical receiver would be 40 dB input variation= 3 dB output variation.
Frank

 

[Ow@i$]

what if I am implementing without an AGC... not necessary should every receiver has an AGC, It is surely a good thing to have but if not then can the opamp be used as filter & amplifier??

 

[BigBoss]

what if I am implementing without an AGC... not necessary should every receiver has an AGC, It is surely a good thing to have but if not then can the opamp be used as filter & amplifier??

In order to filter IF frequency and to obtain large gain, few cascaded stages are used and every stage filters again IF frequency to obtain sufficient selectivity.
OpAmps are technically can be used but they are expensive compare to single transistor.AGC is also possible with OpAmps but more complicated.

 

[chuckey]

Go ahead and build it and see how it performs. You MUST have most of your selectivity before the OPamp otherwise it will cross modulate very badly. So the block diagram is :- RF attenuator -> mixer/oscillator -> IF filter -> opamp-> demodulator-> Audio amplifier.
Frank